1) Field of the Invention
The present invention relates to an airbag device having an airbag, the airbag comprising first and second base cloths sewn at their outer peripheries, and inflated by supplying gas generated by an inflator to the inside of the airbag through a central portion of one base cloth upon collision of a vehicle.
2) Description of the Related Art
Airbags for driver's seats, which are deployed from a central portion of a steering wheel into a vehicle compartment, generally comprises two circular base cloths sewn together at their outer peripheries. Japanese Patent Application Laid-open No. 7-125586 discloses an airbag comprising two base cloths sewn together in V-letter-shaped seams which are spread out radially from the inside to the outside in such a manner that the seams are broken by load generated upon inflation of the airbag, thus preventing the airbag to suddenly expand in fore and aft directions, and allowing the airbag to deploy in fore and aft directions in an overall flat shape in order to softly restrain passengers.
Moreover, Japanese Patent Application Laid-open No. 8-119052 discloses an airbag comprising two base cloths sewn together in breakable circular seams, with a gas intake hole and a gas discharge hole respectively formed radially from the inside to the outside of the seam, thus preventing the airbag to expand at once in fore and aft directions during the initial stage of deployment, and preventing excessive buildup of the internal pressure of the airbag after breaking of the seam by allowing the gas intake hole and the gas discharge hole to communicate with each other.
Incidentally, the airbags described in Japanese Patent Application Laid-open No. 7-125586 and Japanese Patent Application Laid-open No. 8-119052 both include their starting points of break of seams sewn in a V-letter shape, so that the threads on either side of the vertex of the V-letter shape disperse the load of gas pressure which attempts to push and separate the two base cloths, thus inhibiting the smooth break of seams.
Moreover, in the airbag described in Japanese Patent Application Laid-open No. 8-119052, since the seam is formed in a generally annular shape, the airbag inflates in two stages, namely before and after the break of the seam, causing an abrupt change in the airbag's internal pressure and making a smooth deployment difficult.